Members of the armadillo (Arm) family of proteins are homologous to the product of the armadillo gene of Drosophila and have been implicated in a variety of important cell functions. The common characteristic of all armadillo-related proteins is a series of imperfect 42-amino acid repeats (Arm motifs) (Peifer, M. et al. (1994) Cell 76, 789-791). The motif was first described in 1989 in the armadillo locus in Drosophila (Riggleman, B. et al. (1989) Genes Dev. 3, 96-113). Since then, a number of genes encoding proteins containing homologous motifs have been cloned and sequenced. These include: human α-, β-, γ-, δ-catenins; the tumor suppressor adenoma polyposis coli (APC); p120, the substrate of src protein kinases; importin involved in the nuclear import of proteins; and smgGDS involved in the guanine nucleotide conversion of low molecular weight G-proteins such as ras (Hatzfeld, M. (1999) Int. Rev. Cytol. 186, 179-224). Arm motifs are found throughout evolution and are conserved even between distant species like yeast and human. Arm repeats in proteins usually occur in tandem and, so far, no proteins with less than six repeats have been described. It is known that armadillo-related proteins interact with numerous different proteins through their Arm domains (Ozawa, M. et al. (1995) J. Biochem. (Tokyo) 118, 1077-1082; Rubinfeld, B. et al. (1995) J. Biol. Chem. 270, 5549-5555; Troyanovsky, R. B. et al. (1996) J. Cell Sci. 109, 3069-3078; Murayama, M. et al. (1998) FEBS Lett. 433, 73-77).
Recent studies have implicated Arm proteins in Alzheimer's disease. Human Arm-proteins β-, γ-, δ-catenins and p0071 have been found to interact with presenilin-1 (PS1) protein (Zhou, J. et al. (1997) NeuroReport 8, 2085-2090; Zhang, Z. et al. (1998) Nature 395, 698-702; Stahl, B. et al. (1999) J. Biol. Chem. 274, 9141-9148). Mutations of the PS1 and presenilin-2 (PS2) genes are responsible for the majority of cases of early-onset familiar Alzheimer's disease (Price, D. L., and Sisodia, S. S. (1998) Annu. Rev. Neurosci. 21, 479-505). PS1 and PS2 mutations increase the levels of the β-amyloid peptide 1-42 deposited at the core of amyloid plaques suggesting that presenilins are involved in the processing of amyloid precursor proteins (APP). The significance of interaction of catenins with presenilins is unclear, although it appears that wild-type PS1 can stabilize β-catenin, whereas PS1 mutants show loss of the stabilizing function.
As mentioned above, Arm proteins serve different functions in cells. The Drosophila armadillo gene was first identified as a component of the wingless signaling cascade (Wieschaus, E., and Riggleman, R. (1987) Cell 49, 177-184). Similarly, the vertebrate equivalent of armadillo, β-catenin, is known as a critical component of the Wnt/Wingless growth factor signaling pathway that governs cell fate choice in early embryogenesis. The mechanism of how β-catenin transduces the Wnt/wingless signal has been elucidated by the discovery that β-catenin forms a complex with members of the TCF/LEF-1 family of transcription factors that enters the nucleus (Behrens, J. et al. (1996) Nature 382, 638-642; Molenaar, M. et al. (1996) Cell 86, 391-399). TCFs are poor transcriptional activators, but complexes of TCF/LEF-1 and β-catenin act as strong transcriptional activators.
In addition to its role in signaling functions, β-catenin has an essential role in the morphogenesis of solid tissues and the subsequent maintenance of tissue integrity. β-catenin binds to the highly conserved cytoplasmic domain of cadherins and to α-catenin, which binds to actin. The cadherin-catenin complex is a target of regulatory signals that govern cellular adhesiveness and mobility (Kinch, M. S. et al. (1995) J. Cell. Biol. 130, 461-471).
In mammalian cells, β-catenin interacts with the tumor suppressor gene product APC (Su, L. K. et al. (1993) Science 262, 1734-1737). Mutations in APC gene are associated with familial and sporadic colorectal cancer (Kinzler, K. W., and Vogelstein, B. (1996) Cell 87, 159-170). APC is thought to function to decrease β-catenin stability since APC mutant proteins lacking β-catenin binding site display elevated levels of cytosolic β-catenin and constitutive transcriptional activation of the β-catenin/TCF complex.
Thus, Arm proteins are involved in the maintenance of tissue structures as well as intracellular signaling functions. They play a central role in tumorigenesis and are implicated in Alzheimer's disease. Therefore, it is thought that newly discovered members of the Arm family of proteins might be involved in these pathologies and, thus, represent useful targets for the development of therapeutics.
Recent studies from several laboratories demonstrated that insulin-degrading enzyme (IDE) is the main amyloid β peptide-degrading enzyme at neutral pH in rat and human nervous tissues (Kurochkin, I. V., and Goto, S. (1994) FEBS Lett. 345, 33-37; McDermott, J. R., and Gibson, A. M. (1997) Neurochem. Res. 22, 49-56; Qiu, W. Q. et al. (1998) J. Biol. Chem. 273, 32730-32738). Since most of IDE is localized to the cytoplasm, it is unclear how the protease could gain access to amyloid β peptide generated in a secretory organelle from an amyloid precursor protein (APP).